1. Field of the Invention
This invention relates to an image processing method for processing a multi-gradation image input by an image input means such as a camera, an image printing method for printing a multi-gradation image processed by the image processing method on a recording medium, an image printing mechanism using the image printing method and a printed matter printed by the image printing mechanism, for example.
2. Description of the Related Art
Recently, as information has become more electronic and the Internet more popular, more importance is being attached to the technique of using an digital watermark, digital signature and the like in order to prevent forgery and alteration of images. Particularly, the digital watermark technique is proposed as measures against illegal copying, forgery and falsification of a personal authentication medium such as an ID card and a photograph having copyright information embedded therein. The digital watermark technique is a technique for embedding additional sub-information (sub-image information) in a main image (main image information) in an invisible state.
For example, a method for embedding preset information into image information output to a printed matter by use of characteristics of color-difference components and high-spatial-frequency components which are difficult to be perceived by a human being is described in Jpn. Pat. Appln. KOKAI Publication No. H9-248935.
Further, the digital watermark technique which can make it possible to confirm sub-information by use of an optical filter is described in Jpn. Pat. Appln. KOKAI Publication No. 2001-268346.
In the conventional digital watermark technique, a method for embedding sub-information into a main image by mainly using the following characteristics 1 to 3, below, is proposed.
(1) The visual characteristic of a human being is used:
For example, as the visual characteristic of a human being, the factor that the gradation identification ability of the human being becomes lower as the frequency of the image becomes higher and the factor that color-difference information is more difficult to be determined by the human being in comparison with luminance information are provided.
(2) The relation between complementary colors is used:
For example, the factor that “red+cyan” makes an achromatic color is used. In the case of additive color mixture, red and cyan (=green+blue) are set in the complementary color relation. Therefore, if a red pixel and a cyan pixel are adjacent to each other, the combined pixels look like an achromatic color pixel to human eye.
(3) The complementary color relation and color-difference information are used for a high-frequency carrier pattern image:
For example, when red-rich pixels and cyan-rich pixels are repeatedly arranged by use of a high-frequency carrier pattern image, it is impossible for the human eye to identify a difference between the fine color differences. As a result, a human being recognizes that the color difference amount is “±0”.
Further, the sub-information embedded by use of the digital watermark technique is restored by a restoring process by use of key information. For example, when sub-information is restored from an image (a synthesized image having sub-information embedded therein) printed on a printed matter, first, the image printed on the printed matter is read as read image data by use of an image input device such as a scanner or digital camera. Then, the read image data is subjected to a filtering process by a digital filter having a frequency component corresponding to information of the particular frequency of key information. As a result, sub-information embedded as color-difference information is restored.
However, in the above conventional watermark technique, the restorability of sub-information may vary depending on the color of the main image. For example, when the main image is a face image, the image quality of a black portion (region of high concentration) such as a black hair portion may be deteriorated in the printing process by use of the printing mechanism such as a printer or in the image reading process by the image reading device such as a scanner. That is, in the region of high concentration, if sub-information is embedded with the same color-difference amount as that of a portion (region of intermediate concentration) of skin color, the restorability of the sub-information may be degraded in comparison with the region of intermediate concentration in some cases.